1. Field of Invention
This invention relates to protection circuits and more particularly to radio frequency (RF) protection circuits for RF amplifiers or like active circuits.
2. Description of the Prior Art
Several methods are used today to protect amplifiers or other active circuits against failures which cause a loss of signal continuity. Redundant amplifiers coupled together at the input and output ports with hybrids or switching circuits are commonly used to protect against a single amplifier failing. If switching circuits are relied upon to provide signal continuity, some form of performance-sensing device with driver circuits to activate the switches are also required. These circuits provide some measure of protection; however, they are expensive and, like the circuit being protected, they are also prone to failure. The power supply driving the active circuits, which includes the protection circuits, is usually protected against failure by power supply redundancy in conjunction with steering diodes. Standby batteries and/or auxiliary generators are also occasionally used.
Relay switches have also been commonly used (double pole, single throw) at an amplifier input and output to provide signal protection. A short circuit path for signal continuity is provided around the amplifier during periods of amplifier failure. Such an arrangement still requires some form of sensing circuit (such as DC bias current or pilot signal level) plus a driver circuit.
FIG. 1 shows yet another arrangement which is used in the microwave frequency band to protect against the failure of a single reflection-type one-port amplifier. The input signal is supplied on path 10 and coupled through circulator 13 to the reflection amplifier 14. Such an amplifier would typically be a tunnel diode amplifier. The amplified signal is returned via path 12 to circulator 13 and conducted out at the output path 11. In the protected mode, the amplifier input goes to an open circuit impedance which offers a poor termination. The signal is simply reflected back to the circulator and out the output path 11. Such an arrangement is not completely effective, however, since the input termination is not always a good open circuit. Consequently, the signal is attenuated more than what would be desired in the protected mode.
An object of the present invention is to provide a new and improved protection circuit in which a two-port microwave amplifier or like circuit is protected against power supply failure by circuit means which operate in response to a drop in the supply voltage or current.